1. Field
The disclosed embodiments relate to a device for controlling an electromechanical actuator such as a brake piston electromechanical actuator for an aircraft wheel in a system such as an electrically controlled braking system.
2. Brief Description of Related Developments
Electromechanical devices such as aircraft brake systems controlled by electromechanical actuators require that an instruction be transmitted from an operator, here the pilot depressing the brake pedals, to a mechanical system, here the brakes, and that operating parameters of the system be controlled at various levels of the control chain of the system so as to comply with the parameters of the instruction.
In the case of a braking system, the parameters of the instruction are the force on the pedal and its depth of depression which must correspond at the brake level to a greater or lesser loading as a function of the desired braking.
In such an electrically operated braking system, it is known to use for the actuator an electric motor controlling the position and the loading applied to one or more braking pistons acting on a series of brake disks.
At the brake regulation level, speed sensors for example of tachometric generator type make it possible to inform the computer controlling the braking of the speed of each wheel so as to command the optimum braking. When this speed is less than a calculated reference speed, the computer opens a servocontrol limiting the torque at the wheels so as to prevent them locking and to achieve an anti-lock system.
In the case of electrically controlled brakes, electromechanical actuators replace the traditional hydraulic pistons.
The elimination of any hydraulic energy will be fully ensured by virtue of the electrical energy provided by the aircraft and retransmitted to the brakes by electrical boxes which control the pressure loading on the brake disks through electromechanical actuators which themselves replace the traditional hydraulic pistons. Thus, when the pilot brakes, the computer sends the information to a control box, which transforms the electrical information into an electromechanical load. The electromechanical actuators, four in number per wheel, placed on the brake ring, then squeeze the disks together as in traditional hydraulic braking.
A braking system with electromechanical actuators is for example described in document WO 2006/025905 A1.
It is moreover known to couple a device for measuring motor position and speed to the electric motor of such a system so as to allow feedback or slaving of the control system.
The sought-after aim when designing an actuator control is to obtain smooth rotation regardless of speed, as well as fast accelerations and decelerations while having total control of the torque of the motor at zero speed. For this purpose, it is necessary to have information regarding the position and speed of the rotor of the motor.
Various devices for measuring speed and position of a motor are known, coding wheels, resolvers or the like.
It is becoming ever more important to know the parameters such as speed, position, pressure and temperature, but the use of mechanical sensors is less and less desirable. Specifically, the economic factor which requires inexpensive and unimpaired maintenance of the quality of the motor control, demands a reduction in the number of electrical sensors and the frequent elimination of mechanical sensors. Moreover, one of the important problems in the design or use of a sensor of the resolver type and the like is the sensitivity of the sensor with respect:
to the value of linearity and of its frequency of variation that is to say the passband of the sensor,
to time, on account of the aging of the sensor,
to the action of physical quantities of its environment, which are not the subject of measurement and that are called “the disturbances” such as magnetic fields, temperature, humidity, vibrations, shocks, possible variations in the supply voltage.
The electric motors used in systems comprising an actuator operated by a motor are generally brushless motors or direct current motors with no commutator. They consist of a stator made of a stack of laminations in which is disposed a winding generally three-phase star-connected and of a rotor formed on an assembly of laminations and magnets creating the inductor flux.
To avoid the use of sensors, it is necessary to develop efficacious techniques making it possible to implement indirect detection of the speed and/or position of the rotor of the motor of the electromechanical actuator but also ensuring system performance which is very sensitive to variations in its operating parameters and to the accuracy of the measurements.
The high-performance digital controls of synchronous motors with permanent magnets are based on the use of the Park transformation. This state transformation offers a new benchmark in which the electromagnetic torque is a direct image of the quadrature component (q) of the stator current.
However, these controls remain contingent on a measurement of the position and speed of the motor by a dedicated sensor.
As sensors, resolvers are much used in industry to operate such motors but are too expensive.
It is thus very beneficial to estimate the position of the rotor of the motor in an indirect manner.